Transistors power stages

ABSTRACT

In circuitry comprising a switching power output transistor stage and a driver transistor circuit, energy is stored in a magnetic circuit during the transistor on time to provide the turn off current. First and second magnetically coupled inductors are connected in series on the output of the driver transistor, and the base of the power transistor is connected to the junction point of the two series inductors. One or more diodes are connected in parallel with the first inductor to limit the potential drop across this inductor at turn on, and a parallel combination of at least one diode and a resistor is connected in series with the second inductor. The output stage may be two power transistors connected in a Darlington type configuration with a similar inductive circuit provided for each transistor.

This invention relates to new driving arrangements for power transistorsto enable very high efficiencies to be achieved.

With transistors now capable of switching several hundred amperes, thedrive requirements become somewhat more difficult. Transistors requireequal turn on and turn off base currents that now amount to some tens ofamperes, which involves providing both positive and negative suppliescapable of handling these currents, with the consequent losses.

One method which has been used to reduce the turn on base current is theDarlington connection of a driver to the power stage, the advantagebeing that the turn on base current is reduced to about one tenth, i.e.a few amperes, but with the following disadvantages:

(1) Increased losses in the main path (often in the main transistor) dueto the higher saturation of the base-emitter junction plus the extrasaturation voltage of the collector emitter of the driver transistor.

(2) The opposite polarity turn off bias supply is still required at thefull drive capability.

(3) A high voltage driver transistor is required.

The alternative is to use a low voltage driver stage supplying all thecurrent required, thereby eliminating the extra losses of the Darlingtonconfiguration. However, with conventional circuitry, supplying equalturn on and turn off currents still gives rise to considerable lossesand/or additional complexity of drive circuits.

The present invention provides a solution which minimizes both the powerlosses and the circuit complexity, whilst maintaining optimum driveconditions for the main transistor. It is estimated that efficienciesgreater than 99% can be achieved with this new arrangement at verylittle increase in cost.

According to the invention, energy is stored in a magnetic circuitduring the on time to provide the turn off current; hence only one lowvoltage supply is required. The arrangement is such, however, that theon state losses are virtually unaltered; in fact all the currentsupplied by the driver is used to turn the main transistor on, therebyminimising the size of the driver transistor.

The nature of the invention will be better understood from the followingdescription given with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a known Darlington circuit and,

FIGS. 2 to 6 are circuit diagrams of five embodiments of the invention.

Referring firstly to FIG. 1 this shows a known Darlington circuit. Tr₁is the main power transistor, with Tr₂ as the driver, deriving itscollector supply from the main power circuit Vs. Whilst the drivercollector current Ic₂ adds to the load current Ic₁, the main powertransistor Tr₁ cannot saturate due to the higher Vbe₁ plus the drop inTr₂ of Vce₂ (sat). Typically these two voltages are 1.5 to 3 V for Vbe₁and 1.7 to 2 V for Vce₂ (sat). Thus the static power losses in Tr₁ areP_(c) +P_(B) =I_(c).sbsb.1 +Vce+IB₁ ×Vbe which typically amount to200×(2.5+1.7)+20×2.5=890 W. This has to be dissipated (as heat) by meansof large `heat sinks` into the cooling media (usually directly into thesurrounding air). Also the auxiliary 5 V negative supply has to becapable of providing the IB₁ turn off current. A common arrangement isas shown from which it is obvious that considerable power can be wastedin the base resistors during the on state.

Typically R_(B1) would be of the order 0.25Ω (in order to provide a -IB₁of 20 A from the -5 V auxiliary supply (limited to this value by thetransistor reverse Vbe₁ characteristics). Hence in the on state whenVbe₁ is positive by some 2.5 V there is 7.5 V across the 0.25Ω whichtherefore takes 30 A and dissipates 225 W.

Thus Tr₂ has to provide not only 20 A drive into the base of Tr₁ butalso 30 A into R_(B1). Hence Tr₂ must be capable of handling 50 A, andsince it derives its supply from the collector circuit of Tr₁ this mustalso be a high voltage device and hence expensive.

Similar arguments about losses apply, on a reduced scale, to the basecircuit of Tr₂.

Thus for ±5 Amps IB₂ drive (assuming H_(FE) of 10 at Ic₂ of 50 A), thepositive +10 V auxiliary supply has to provide 15 A (10 A goes intoR_(B2) (1Ω) due to Vbe₂ +Vbe₁ ≈5 V.

Hence total losses in a Darlington pair and associated resistors amountsto some 1300 Watts.

At present 200 Amp transistors are available at some 100 V rating, i.e.20 KW load; therefore 1300 W loss represents a significant (61/2%) partof the load (especially if the power source is a battery as in anelectric vehicle) i.e. Efficiency≈93.5%).

FIG. 2 shows a circuit arrangement according to the invention.

In this arrangement, Tr₁ is driven into saturation by all the currentfrom Tr₂ (D₁ [shown as one diode but may be two or more in series] diodeVF to be≧Vbe of Tr₁). The switching input ZA is connected between groundand a resistor R₃ the other terminal of which is connected to the base(control electrode) of transistor Tr₂. The emitter (common electrode) ofTr₂ is connected to the positive auxiliary supply via resistor R₂ and azener diode V_(z) is connected between said supply and the base oftransistor Tr₂. Drive current I₂ flows in the resistor R₃. During the onperiod energy is stored in the coupled inductor L₁ -L₂ by currentflowing through winding L₂, the current being controlled by somemeans--a resistor etc. or as shown by the driver transistor Tr₂ used asa current source by virtue of feedback in its emitter circuit, the basepotential being controlled by V_(z) in the on condition. Diode(s) D₂limit the potential drop across L₂ at turn on to avoid the delay whilstthe current is established in the inductor.

At turn off of the transistor Tr₁ and Tr₂ the inductor tries to maintainthe same current flowing and in an endeavour to do this the potentialacross it reverses rapidly. Tr₂ is now non-conducting but since L₁ istightly coupled to L₂, diode(s) D₁ are forced into conduction, sokeeping the current flowing in the inductor but now in a reverseddirection in the Tr₁ base-emitter circuit.

The current in the inductor gradually decays to zero, governed by thetime-constant of the circuit, which is chosen to be long enough toensure that Tr₁ is fully turned off (typically a few micro seconds)after which time the main power transistor is held off by R₁ providing apath for the small leakage currents (R₁ typically 10 to 100Ω).

The losses in this circuit are typically as follows.

    Tr.sub.1 Vce sat 2 V @200 A=400 W.

(newer transistors are coming along with Vce sat less than 1 V; hencethe importance of being able to work with them saturating).

Drive circuit Aux supply 5 V @22 A=110 W.

Total losses 510 W. (1300 W for Darlington).

Hence overall efficiency improved to≈99%.

Also Tr₂ is only a low voltage device therefore cheaper.

The diode D₃ is necessary in practice to prevent negative voltagebreaking down the base-emitter junction of Tr₁ on reverse bias.

The inductor coils L₁, L₂ will have the same number of turns, but notnecessarily the same gauge wire, and may be wound on powdered corematerial.

The diode D₂ can be replaced by an RC circuit; except that it isundesirable to introduce a `ring` into the circuitry. The diode D₁ willusually be more than one diode because the transistor potential drop isquite large. It is not quite true to say that all the current of the L₁,D₁ circuit goes through the base of the main power transistor becausethere is a little leakage through the parallel resistor R₁ ; however,substantially all the current flow is through the main power transistor.

The same principal can be applied to a Darlington connected outputstage, as shown in FIG. 3. In this case the transistor Tr₁ is replacedby two transistors Tr_(1a) and Tr_(1b) connected as a Darlington pairand each having a base circuit comprising inductors L_(1a), L_(2a) orL_(1b), L_(2b), resistor R_(1a) or R_(1b) and diodes D_(1a), D_(2a),D_(3a) or D_(1b), D_(2b), D_(3b), connected as in the base circuit oftransistor Tr₁ of FIG. 2. A modification of this circuit is shown inFIG. 4, where the inductors L_(1b), L_(2b) and diode D_(2b) oftransistor TR_(1b) are replaced by an inductor L_(1s) connected betweenthe emitter of Tr_(1a) and base of Tr_(1b) and diodes D_(1b), and aninductor L_(1p) with a diode in parallel is connected between thecollector (output electrode) of Tr₂ and the inductors L_(1a), L_(2a),L_(1p) will have nominally 10×number of turns of L_(1s), correspondingto Tr_(1a) having a gain of 10.

FIG. 5 shows a modification of the circuit of FIG. 2 in which aninductor L₃ is used in series with the output load to limit the turn oncurrent to a permissible value within the safe operating area rating(S.O.A.R.) of the main power transistor Tr₁. The hole storage effect inthe flywheel diode D₅ in parallel with the load, necessary with anyinductive load, causes the anode of D₅ to remain effectively at supplypotential after it has been forward biased, until all the carriers havebeen removed by reverse current. Hence when the transistor Tr₁ is turnedon, if the current were not limited in some way it would try to shortthe supply and the transistor Tr₁ and/or the diode D₅ could be damagedor destroyed. This is prevented by the inductor L₃. However, the energystored in inductor L₃ has to be dissipated on turn off and soseries-connected diode D₆ and resistor R₅ (which could be the resistanceof the windings of L₃) are provided in parallel with the inductor L₃.

The R.M.S. rating of diode D₆ is much less than that of diode D₅ andhence a smaller diode is used and the hole storage is much less so thatthe current peak is narrower and usually less in amplitude.

The capacitor C₁, diode D₄ and resistor R₄ form a "snubber" network tocontrol the operating locus during turn off. Such networks are wellknown in use with both silicon-controlled rectifiers and transistors.

Referring to FIG. 6, when very large currents are being controlledand/or when efficiency is of paramount importance, e.g. in batteryelectric vehicles, energy can be recovered by using an overwinding(secondary winding) W on inductor L₃ having a much larger number ofturns and connected across the supply battery Ve in series with a diodeD₆.

It will be understood that, whereas NPN transistors have been shown, PNPtransistors can equally well be used. Also the load can be in theemitter circuit.

I claim:
 1. A transistor power switching stage and driver circuitcomprising a driver transistor having first and second output electrodesand a control electrode, a power transistor having first and secondoutput electrodes and a control electrode, a switching control circuitconnected to the control electrode of said driver transistor, first andsecond magnetically coupled inductors connected in series electricallyto said second output electrode of said driver transistor, a connectionbetween the junction of said series-connected inductors and the controlelectrode of said power transistor, and at least one diode connectedbetween said second output electrode of said driver transistor and saidcontrol electrode of the power transistor.
 2. Circuitry according toclaim 1, wherein the second inductor is connected to a driver transistorsupply line in series with a parallel combination of at least one diodeand a resistor.
 3. Circuitry according to claim 2, wherein an oppositelypoled diode is connected in parallel with the second inductor and saidparallel combination.
 4. Circuitry according to claim 1 or claim 2 orclaim 3, wherein the inductors have the same number of turns and arewound on powdered core material.
 5. Circuitry according to claim 1 orclaim 2 or claim 3, wherein a second power transistor is connected tothe first in Darlington configuration, the same inductive circuitarrangements being provided between the two Darlington connectedtransistors as between the first such transistor and the drivertransistor.
 6. Circuitry according to claim 2, wherein a second powertransistor is connected to the first in Darlington configuration, athird inductor is connected in series between the output electrode ofthe driver transistor and the first inductor, a fourth inductor isconnected between an output electrode of said first power transistor andsaid supply line, and the number of turns in the third inductor isgreater than the number of turns in the fourth inductor by a factorsubstantially equal to the gain of the first power transistor. 7.Circuitry according to claim 6, wherein a diode is provided in parallelwith said third inductor, and a parallel combination of one or morediodes and a resistor is provided in series with said fourth inductor.8. Circuitry according to claim 1, further comprising a flywheel diodein parallel with a load connected to the first output electrode of thepower transistor, and a load inductor in series with said load to limitthe turn on current.
 9. Circuitry according to claim 8, wherein a diodecircuit is provided in parallel with the load inductor.
 10. Circuitryaccording to claim 8, wherein a snubber network, comprising a resistor,capacitor and diode, is connected across the output electrodes of thepower transistor to control the locus of operation during turn off. 11.Circuitry according to claim 8 or claim 10, wherein an overwinding isprovided on said load inductor having a much larger number of turns andconnected in series with a diode across the load supply.
 12. Atransistor power switching stage and driver circuit comprising a drivertransistor having first and second output electrodes and a controlelectrode, a power transistor having first and second output electrodesand a control electrode, a switching control circuit connected to thecontrol electrode of said driver transistor, first and secondmagnetically coupled inductors connected in series electrically to saidsecond output electrode of said driver transistor, a connection betweenthe junction of said series-connected inductors and the controlelectrode of said power transistor, and at least one diode connectedbetween said second output electrode of said power transistor and endterminal of said series-connected inductors remove from said secondoutput electrode of said driver transistor.